Ratios & Proportions
Adding and subtracting fractions
Cross-multiply to reach a common denominator, combine the numerators, and simplify.
The Formula
a/b ± c/d = (a x d ± c x b) / (b x d)
Cross-multiply to reach a common denominator, combine the numerators, and simplify.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| a, c | Numerators | count | The counts above the line. |
| b, d | Denominators | count | How many parts make a whole. |
What Each Symbol Means
a, ccount
Numerators
The counts above the line.
b, dcount
Denominators
How many parts make a whole.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate fractions page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate Fractions, filed under Math. Every number the guide produces comes from substituting into this one line.